This invention relates to an apparatus for sensing the passage of documents past a sensing location.
The invention has application, for example, to a depository apparatus included in an automated teller machine (ATM) of the kind which is arranged to carry out a financial transaction, such as dispensing currency notes or accepting a deposit of money, as may be required by a customer. As is well known, in operation of an ATM of this kind, a user inserts a customer identifying card into the machine and then enters certain data (such as a personal identification number, type of transaction, and quantity of money required or to be paid in) on one or more keyboards included in a user console of the machine. The machine will then process the transaction, dispense currency notes or accept a money deposit as may be requested, and then return the card to the user as part of a routine operation. If money is to be deposited, the user typically inserts an envelope containing the money (cash and/or cheques) through a deposit entry slot in the user console, and the depository apparatus of the ATM transports the envelope to, and deposits it in, a container included in the apparatus. In order for the operation of the depository apparatus to be controlled correctly, it is necessary to provide sensors for sensing the passage of an envelope past certain points, hereinafter referred to as sensing locations, along a transport path for the envelope between the entry slot and the said container. For example, the operation of a printer may be initiated when the training edge of an envelope passes a sensor.
A known apparatus for sensing the passage of documents past a sensing location includes a light responsive device, such as a phototransistor, operably associated with a light emitting device, such as a light emitting diode (LED). The sensing location is defined as the position in the transport path for the documents where the leading edge of a document interrupts the passage of light from the light emitting device to the light responsive device. The light responsive device forms part of voltage generating means the output voltage of which has a first (high) value when a document is not present at the sensing location, that is to say when the passage of light from the light emitting device to the light responsive device is not interrupted by a document, and which has a second (low) value when a document is present at the sensing location, that is to say when such passage of light is interrupted by a document. The output of the voltage generating means is applied to a peak detector formed by a capacitive voltage storage means which provides an output voltage whose peak value is equal to said first value and whose value progressively decreases while a document is present at the sensing location. The apparatus includes voltage comparison means for comparing the output of the voltage generating means with the output of the capacitive voltage storage means and for providing a first or a second output signal indicative of whether or not, respectively, a document is present at the sensing location, depending on whether the output voltage of the voltage generating means is less than or greater than, respectively, a predetermined fraction (e.g. 80%) of the output of the peak detector.
By virtue of the use of the peak detector and the voltage comparison means, the known document sensing apparatus described above has the advantage that it will continue to provide a first (high) signal when a document is not present at the sensing location, even when the output voltage of the voltage generating means with no document present has fallen over time due to dust building up on the light emitting and light responsive devices and/or due to aging of the light emitting device. However, this known apparatus has the disadvantage that if a document is present at the sensing location for an abnormally long time, for example due to a jam occurring, the output of the capacitive voltage storage means may fall to a value such that the voltage comparison means provides a second output signal, falsely indicating that the document has cleared the sensing location.